The book entitled "High-Frequency Circuit Design" by James K. Hardy, Preston Publishing Company, 1979, pp. 317-318, describes mixer circuits. The FIG. 10-6(f) of this book shows a mixer circuit having a double-balance structure (double balanced mixer). The circuit includes four diodes which are connected so that they form a ring. The diode ring receives a radio-frequency signal (RF) and an oscillator signal (OSC) via a transformer. An intermediate frequency signal (IF) is obtained via another transformer. Thanks to the double-balance structure, relatively little remains of the oscillator signal in the intermediate frequency signal.